As part of new studies concerning the low pressure compressors of a turbojet engine, most research work has been directed towards lightening the weight of the rotors of these compressors so as to obtain a gain of weight. These rotors are mainly of the monobloc type and are known in English terminology under the name of "BLISK", namely a contraction of the term "bladed disk". These rotors are monobloc, that is the blades form an integral part of the hub or are linked to the latter, as opposed to "winged disk" type rotors whose blades are provided with feet engaged in machined alveoles at the periphery of the hub before being locked in said alveoles.
So as to lighten these "blisk" type rotors, two technological solutions have been examined. One first solution consists of machining the turbine blade in the block, that is when the blades are embodied in situ at the periphery of the disk by machining the blade profiles. These profiles thus form an integral part of the hub. They are then recessed, also by machining. However, this technological solution is difficult to implement owing to the small thickness of the blades and complex twisted shape. This solution is far too costly to be produced on an industrial scale.
A second solution consists of preparing the blades previously recessed by machining and twisted and which are next solely linked to the hub or boss by various techniques, such as welding or soldering. This solution, much easier to implement and less expensive industrially, has been retained.
Various techniques for securing blades to a hub are already known.
According to the patent application FR-A-2 602 26, there exists a method for producing a set of rotors for a gas turbo-engine. This method firstly consists of producing a disk fitted with two external annular flanges, and secondly a ring of blades, also provided with two annular flanges, and then of hot-assembling the ring of blades on the disk. Then the flanges are sealed under vacuum and the entire unit is subjected to hot isostatic compression.
This method can only be applied to full blades or at least blades including a full base and two lateral flanges since it consists of crushing the two faces opposite the flanges of the blade and those of the disk so as to form a continuous link. This method is unable to correctly fix the hollow blades whose cavity opens at the base of said blade.
According to the patent EP-A-O 458 630, there is also a method to secure individual blades to the disk of a "BLISK" type rotor. This method is applicable to full blades which are retained between the two jaws of a fixing device and linked on the disk by a linking operation by means of friction. This method is also applicable to the repair of blades.
According to the pat. U.S. No. 4 034 182, there is also a method consisting of assembling via their lateral sides a series of slightly concave segments bearing on their outer surface a compressor blade so as to form a ring fitted with radial blades once these elements have been assembled. However, this method is difficult to apply for the embodiment of "BLISK" type rotors with hollow blades.
Finally, according to the patent application FR-A-2 619 331, there exists a method for producing a rotor with full blades, especially for a gas turbine engine. This method concerns the securing of the full blades to the disk of a rotor. It includes stages consisting of firstly forming a cavity in a portion constituting the foot of a blade, and secondly a plurality of cubic projections on the periphery of the disk of the rotor. Then each blade is assembled by being nested on the disk. The actual fixing is effected by means of pressure and heating. This link is mainly effected between the upper face of the cubic projection and the bottom of the cavity provided in the blade. Subsequently, it is possible to carry out a machining so as to remove the portions of the blade which define the lateral portions of this cavity. This type of method cannot be applied to the entire height of hollow blades since the extremity surface of the cubic projection is then unable to abut against the bottom of the cavity of the blade.
Finally, as shown on the attached FIG. 1, when it is desired to weld a hollow blade 1 to a rotor disk 3, the weld seam 5 exhibits a weld projection 7 inside the hollow blade and a projection 9 outside the latter. Now, it is impossible to mechanically machine the internal projection 7 owing to the extreme thinness of the blade 1 and its twisted geometry. As a result, the state of the internal surface of the blade is not compatible with non-destructive inspection tests (radiography, ultrasounds, etc) absolutely essential for observing aeronautic requirements.